EasyChair Preprint№ 5126

Overview of Nano-Fiber Fabrication viaElectro-Spinning Tailored for Energy StorageSystems

Amirhossein Ahmadian, Mohammadali Alidoost, Abbas Shafieeand Amin Akbari

EasyChair preprints are intended for rapiddissemination of research results and areintegrated with the rest of EasyChair.

March 9, 2021

Overview of nano-fiber fabrication via

electro-spinning tailored for energy storage systems

Amirhossein Ahmadian1, Mohammadali Alidoost2, Abbas Shafiee3,and Amin Akbari3

1Department of Mechanical and Aerospace Engineering, University of California, Los Angeles,CA, 90095, USA;2Department of Bioengineering, University of California, Los Angeles, CA 90095, USA;3School of Mechanical Engineering, Purdue University, West Lafayette, IN, 47907, USA

E-mail: aahmadian@ucla.edu

Abstract. The ultra-fine fibers arising from electrospinning have two significant properties,such as a large surface to volume ratio and a structure that is relatively free from defects ata molecular level. A high surface to volume ratio makes electro-spun materials suitable forundertaking activities that require a higher physical contact, including providing a site for achemical reaction as well as the filtration of small-sized physical materials. The fibers providesufficient storage space and also play a critical role in converting the stored energy into electricalcurrents by easing the transfer of electrons. However, electrospinning possess various limitations,including difficulties in preparing inorganic nanofibers and limited quantity or variety ofpolymers used in the process. In this analytical study, the nano-fiber bundles’ fabricationprocess with electrospinning and correlation between extrinsic electrospinning parameters andrelative abundance of different resulted fiber morphologies tailored for energy storage systemsis examined.

1. Review on Electrospinning and Electrostatic PhenomenonIn the early 1930s, Formhals employed electrospinning fabrication method as a fiber spinningtechnique. In 1934, the invention was patented, titled as” process and apparatus for preparingartificial threads” which developed spinning techniques. However, some practical issues such asfiber drying and collection had remained until he initiated his first patent overcoming spinningdifficulties at the time. A movable thread was applied to collect the threads in a stretchedcondition. In his patent, he reported the spinning of cellulose acetate fibers utilizing acetone asthe solvent [1, 2].

Electrostatic phenomena arise from the ability of electrons to move with relative ease invarious materials. Electrostatics comprises of two general classes including conduction andinduction. Induction refers to a temporary state where electrons in a substance are eitherattracted to the repelled by the nearby charged object [3]. Conduction, on the other hand,occurs when a charged object comes into actual contact with a neutral object. The excesselectrons move from a charged object to a neutral object; thus, when the objects are separated,the objects acquire the same charge [4]. Electrostatic charges exert forces calculated usingCoulomb’s law F= k Q1.Q2/d2 between opposite charges causing water droplet deformation [5].In general, electrospinning refers to the production of fibers by means of electric current to

draw charged threads of polymer solutions. The fibers produced using this process usually hasa thickness of hundreds of nanometers. The electrospinning process shares the characteristics ofconventional dry spinning as well as electro-spraying of fiber [6]. The process is highly suitablefor the production of complex and large molecules as the process does not involve the use ofchemistry coagulation or the application of the high temperatures [7–12].

2. Overview of Nano-fiber Bundles Fabrication Methods and Electro-spinningNanofibers refer to fibers that have a diameter of less than 1000nm; they can be developedusing various processing techniques [7,8]. So far the nano-fiber making techniques include directdrawing [13–15], magneto-spinning [16], extrusion [17, 18], melt-blowing [19], hard templating[20], soft-templating [21], self-assembly [21,22], lithography [23,24], centrifuge spinning [25,26],hydrothermal/solvothermal [27], ball milling [28], chemical vapor deposition [29, 30], andelectrospinning [31–34]. Among thereof fabrication methods, electrospinning outperforms dueto its many advantages such as controllable fiber diameter (from tens of nanometer to a fewmicrons), covering fabrication of wide range of materials (natural and synthetic polymers, metals,ceramics, composites, sol-gels), versatile fiber morphologies (porous, dense, core-sheath, hollow,spiral, side-by-side, nanoparticles, nanorods, nanowires, nanosheets, and nanobelts), and capableof large scale production [7–11].

Nano-fibers formation stems from the electrostatic force along with the spinning forceresulting in the continuous splitting of polymer droplets. Nanofibers deposit on the metalcollector plate, layer upon layer, thus resulting in the formation of a nanofibrous mat [35–37].The extrinsic parameters of the electrospinning process significantly impact the nanofiberstructural morphology. Extrinsic parameter comprises of the working distance, viscosity,conductivity, polymer solution, humidity, temperature, as well as the applied voltage. To attaina uniform nanofiber mat, it is crucial to optimize the extrinsic parameters. When the nano-fibrous mat is uniform, it results in the formation of the bead-containing fibrous structureand continuous fibrous structure (nanofiber yarns). Nanofiber yarns are defined as entangledcontinuous fiber bundles possessing two intrinsic features: continuous length and interlockedtwisted structure [38].

The correlation between extrinsic electrospinning parameters and relative abundance ofdifferent fiber morphologies leads to the formation of different nanofiber bundles forms [39].In general, nanofiber bundles can be categorized as the following [40] (See Figure 1):

Figure 1: Nanofiber bundle categories.

3. Electrospinning CategoriesNanofibers production by means of electrospinning technique may occur in two ways needle-lessand needle-based. Needle-based electrospinning constitutes starting with a polymer solution in

a tightly closed reservoir as these limits as well as prevents solvent evaporation. The needle-based system is essential as it allows for the processing of a wide range of materials, includinghighly volatile solvents [41]. Needle-based electrospinning has the following advantages processflexibility where it has the capacity to process various structures such as multi-axial and core-shellfibers. The distinction between the two fibers enables the incorporation of Active PharmaceuticalIngredients (API) to be incorporated in the fiber. Another advantage is that the needle-basedmethod has tightly controlled flow rate, minimizes solution waste, and has a limited number ofjets [42]. The Numerous advantages have made the needle-based method immensely popular.

On the other hand, needle-less electrospinning allows for the large-scale production ofmaterials. A Starting polymer solution inside an open vessel is utilized to generate nanofibersusing a rotating or stationary platform [43]. However, the needle-less electrospinning methodcannot carry out versatile fiber production. Also various process parameter cannot be controlled,including the flow rate [44].

4. Electrospinning Process and PrinciplesElectrospinning refers to a process used to develop a nonwoven fabric that is impermeable usingsubmicron fiber when a liquid jet that is a millimeter in diameter is pushed through a nozzlethat has an electric field. In general, the fiber formation process in electrospinning can beobserved and classified into three different stages: Deformation of the prolate droplet (Talyorcone) and jet initiation, whipping or bending instability, and fiber deposition. The general setupfor electrospinning is depicted in Figure 2. The electrostatic charging at the tip of the nozzle iscrucial to the formation of a Taylor cone where a single jet of fluid ejects [36]. The accelerationand thinning of the jet in the electric field and radial charge repulsion causes the primary jetto split into multiple filaments; this is referred to as “Splaying.” The size of the resultant fibersdepends on the number of the subsidiary jets formed. Under normal conditions, the fluid jetwhipping in electrospinning is immensely fast as this condition is essential for the production ofnanofibers [45]. In general, the fiber formation process in electrospinning can be observed and

Figure 2: General electrospinning setup.

classified into three different stages: Deformation of the prolate droplet (Taylor cone) and jet

initiation, whipping or bending instability, and fiber deposition.

4.1. Taylor ConeIn 1964, Geoffrey Taylor in initially described the cone. Taylor’s primary interest was indetermining how water droplets behave within strong electric fields, for instance, thunderstorms.Exposing a small volume of a liquid that is electrically conductive to electric field results in shapedistortion due to surface tension. Increase in voltage increases the impact of the electric field,as the electric field exerts a magnitude force on the droplet similar to the surface tension resultsin the formation of a cone shape. On reaching a given threshold voltage, the rounded tip invertthen releases a jet of liquid. The cone-jet commences the start of the electro-spraying processachieved at a voltage higher than the threshold. The Taylor cone refers to the theoretical limit ofa cone-jet when the electro-spraying process commences. For a perfect cone to be achieved thereneeds to be a semi-vertical angle of 49.3 degrees, the cone surface needs to be equipotential, andthe cone ought to exist in a steady-state equilibrium [46]. Taylor cone formation is an essentialpart of the electrospinning process. Symmetrical vortices arising within the Taylor cone is likelyto increase the velocity of the solution. Beads that occur in cone-jet results in the formation ofbeaded nanofiber [47].

Figure 3: Deformations of the prolate droplet and Taylor cone by increasing the electric fieldcaptured in different timings and ultimately, the formation of the jet [37].

In Figure 3, the formation of a Taylor cone captured in different timings can be observed.

4.2. Whipping and Jet InstabilityA strong electric field is likely to deform a liquid of finite electric conductivity form a conicalshape arising from the balance between the surface tension and electric stresses. Conversely,close to the apex of the cone the structure is unstable and the thin jet structure replaces theassociated singularity. The electrospray arises from the imposed flow rate of the liquid of thecone-jet structure that has stability within certain applied voltage values — the arising from thecone-jet structure breaking into spherical droplets due to axisymmetric instabilities. However, alateral instability causes the jet to bend of its axis arising from electrostatic repulsion between

the straight and the bent parts of the jet. In the instance that the whipping instability growthrate is larger than the one associated with a jet breakup; thus, the jet off-axis movement becomesa significant aspect of its evolution [48].

Replacing a liquid with a polymer solution where the solvent evaporates prior to a dropbreakup taking place results in polymer fiber formation. The existence of lateral instabilitywithin the electrospinning application results in the formation of thinner fibers as the bendingcontinues to stretch and along with thinning the jet. However, in most experiments carried out,the whipping is noticeably chaotic, thus making it difficult to have an in-depth understandingof its properties and structure [7, 8, 48].

4.3. Fiber DepositionNano-fibers formation emanates from the electrostatic force accompanied by the spinningmechanical force resulting in the continuous splitting of polymer droplets. Nanofibers depositon the metal collector plate, layer upon layer, thus resulting in the formation of a nanofibrousmat [35–37].

5. Solution-based Electrospinning and Related Effective ParametersSolution-based electrospinning needs a solvent to solubilize a given polymer. Consequently,identifying the correct solvent plays a vital role in attaining a homogeneous polymer solution.The solution parameter is lucrative in determining a suitable solvent for a given polymer.Solubility parameter takes into consideration the various molecular interactions in a given moleof material including such as polar interaction, dispersion forces, as well as specific interaction,including hydrogen bonding [49]. Cohesive energy is given as

E = ∆H −RT (1)

Where: ∆H is Latent heat of vaporization, T is Absolute temperature, and R is Universal gasconstant.Later, Charles M. Hansen extended the Hildebrand solubility theory to Hansen SolubilityParameters (HSP) 21, which estimates the relative miscibility of polar and hydrogen bondingsystems as [50]:

δi2 = δd

2 + δp2 + δh

2 (2)

where: δi is Hansen solubility parameter, δd is Dispersive component, δp is the polarity, and δhis the hydrogen bonding. A suitable solvent for a particular polymer ought to have a solubilityparameter close to that of the polymer. Therefore, calculating the Hansen solubility parameterwhere the polymer-solvent ought to have a small value of Ra [51].

Ra2 = 4(δd1 − δd2)2 + (δp1 − δp2)

2 + (δh1 − δh2)2 (3)

Additionally, a suitable solvent ought to have a relative energy difference (RED) of less thanone. And RED = Ra/Ro Where Ro refers to the radius of a sphere [51]. where:RED < 1 the molecules are similar and dissolve, RED = 1 the molecules partially dissolve, andRED > 1 the molecules do not dissolve.

Nanofibers fabrication could be affected by many factors which lead to different morphologiessuch as uniform or ordered pattern structure with a round cross-section, beads-on-stringstructures, or individual beads. These crucial factors such as polymer type, polymer molecularweight, polymer distribution, polymer concentration [52, 53], solution conductivity, solutionsurface tension, solution viscosity, and solvent properties. Solvent properties contain boilingpoint, volatility, and dielectric properties [54]. Operating factors are another key parameterin final fiber morphology, which including applied voltage, collecting distance, the flow rate

of the polymer solution [55]. Last but not least, external conditions or ambient conditionsfor intense humidity, ambient temperature, and addition flow are also effective parameters onelectrospinning [7, 8, 33,56].

5.1. ConcentrationThe electrospinning process depends on polymer concentration as the most crucial factor. Suchthat changing in polymer concentration leads to a change in solution viscosity. As the polymerconcentration increases, the viscosity increases first at a steady rate and thereafter at a muchhigher rate [57]. The solution viscosity is extremely governed by intermolecular interactionsbetween polymer-polymer, polymer-solvent, and solvent-solvent within the polymer solution.Electrospinning a dilute polymer solution is analogous to an electro-spraying process. On theother hand, the polymer intermolecular distance within the solution is so considerable thatthe interaction is considered to be very weak. Therefore, the viscoelastic force in the polymerjet is minute to form a uniform fibrous structure. In this process, the jet splits into separatecharged sections as the voltage is applied. Gradually theses charged sections turn into dropletsor individual beads as a result of high surface tension caused by solvent evaporation whilespinning. Similarly, an increase in the polymer concentration leads to higher viscoelastic forceand more difficult for the jet to be split. In lieu of breaking the like-charged sections, electrostaticrepulsion within the solution elongates the links between charged sections, consequently formingthinner filaments. Relatively thicker sections stretch thinner although to the links betweencharged sections. As the solvent evaporates while spinning, due to the surface tension, filamentstend to take the shape of beads-on-string morphology. Accordingly, a further increase in thepolymer concentration, brings about uniform elongation of and formation of the jet, resultingin homogeneous fiber morphology [7, 8, 57,58].

critical concentration (c*) is defined to distinguish whether predominant intermolecularinteraction will happen or not leading to a chain entanglement [59]. In this regard, the followingformula has been utilized to describe solution entanglement. The following formula calculatesthe ratio of polymer molecular weight to the solution entanglement molecular weight [60,61]:

(ne)solution =MW

(Me)solution=φMW

Me(4)

where: Me is entanglement molecular weight, MW is polymer weight average molecularweight, and φ is polymer volume fraction.if (ne)solution < 2: polymer chains do not entangle and individual beads structure is formed.if 2 < (ne)solution < 3.5: insufficient polymer chain entanglement and beads-on-string structureis resulted.if (ne)solution > 3.5: sufficient polymer chain entanglement and beads-on-string structure isshaped.

In general, throughout electrospinning, the solution viscosity can be increased by using aconcentrate polymer solution or increasing the weight of the molecular polymer. For instance,polylactide solution doped with polyethylene oxide that has a high molecular weight increasesviscosity of the polymer solutions [62]. Increased viscosity improves the jet stability allowingfor the construction of multi-filament yarn, individual fiber, and aligned unidirectionally acrossa large area or enabling the individual filaments the develop an ordered pattern. Introducing afiber-forming agent resulted in the formation of an elongated stable jet suitable for the collectingarrays of aligned fibers [63]. Research has shown that using the polymers with a higher molecularweight and increasing viscosity by increasing the concentration of the electrospinning solutionresults in a stable jet [56].

5.2. SolventSelecting a proper solvent plays a prominent role in the electrospinning process. Since solution’sconductivity, viscosity and surface tension are affected by a handful of solvent’s characteristicsincluding solvent’s conductivity, boiling point, vapor pressure, polarity, dipole moment, dielectricconstant, etc.

Solvent volatility ought to match up with the jet’s traveling time, i.e., solvents with lowvolatility result in wet web structure in the end. In the same manner, rapid evaporation ofhighly volatile solvents is accompanied with cooled down and frozen filament surface leading toa porous surface on the web structure [64] (see Figure 4).

Figure 4: (a) solid beads vs.(b) porous beads produced by electrospinning [64].

Electro-spinnability of a solution can be enhanced by doping a solution using salt, using asolvent that has a high dielectric constant and higher conductivity as this increases the spinningjet charge density. However, these same factors result in a shortened length jet that ought to beavoided if one has an objective of attaining acquitting a stable jet with a greater length. Dopingpolyethylene oxide with salt illustrates higher conductivity minimizing the length of the stablejet. However, the chloroform system the critical conductivity between the bending instabilityand the long stable jet is estimated to be as low as 0.6 µS/cm [65].

5.3. Voltage and Electric FieldConventionally, in the electrospinning process, an increase in the voltage leads to an increase inthe length of a stable jet. The elevation in length stems from a larger tangential electric field fromstarting from the needle tip as well as lower static charge density to stabilize the electrospinningjet. However, the conventional effect does not cater to all solutions. The inconsistency may bedue to a variety of factors such as dielectric property and conductivity of the solution. Thesefactors have more influence on jet stability [66].

The bending and stretching stability of the jet are affected by applied voltage in the sensethat uniform patterned web could result at higher voltages in some cases. However, the appliedvoltage is not the only principal element, and other elements such as flow rate and jet travelingspeed, and density have to be compromised at the same time [67].

Typically, the electrospinning process uses direct-current (DC) voltage; however, alternating-current (AC) has been reported in a few papers which are not considered as safe as DC voltage,particularly in high voltage circumstances [68–70]. The reason for taking advantage of AC couldbe witnesses when highly aligned fibers are sought. In this sense, highly aligned web structureis collected on a rotating mandrel when the AC power source is applied [71].

5.4. Flow RateThe solution flow rate has to be reached at a minimum certain point at which the spinningjet can compensate for the solution evaporation rate in order to maintain the continuous flowof the fibers [72]. If the flow rate is higher than needed, the solution will accrue at the tip ofthe needle resulting in a formation of large droplets hindering the formation of normal Taylorcone, and ultimately solution dripping off the tip. Sometimes also blockage of the needle ornozzle happens, which is due to the rapid evaporation of the solvent, causing solidification ofthe droplet inside the needle or nozzle [73,74].

5.5. Collecting DistanceThe collecting distance is proportional to the electric field intensity in the sense that solutiontraveling time can be affected directly. In some circumstances, less collecting distances can behelpful meaning that stronger electrostatic force is applied to the jet consequently shorter timefor the jet to travel. A stronger electrostatic force can be either efficacious due to the effectivestretching of the jet hence the formation of finer fibers on the collector. However, shorter distancemay cause reverse effects, i.e., incomplete solvent evaporation, thereafter formation of wet fiberwebs. Similarly, increasing the distance can lead to a weaker electric field and dripping of thejet in the half-way towards the collector [8, 55,72,73].

5.6. PolarityThe electrospinning process with high voltage electrode applied to the nozzle causes more strongand concentrated electric field on the nozzle such that when the jet travels the distance fromthe high charged needle to the collector, it experiences a reducing electrostatic force. Thisfacilitates the chaotic whipping instability, which brings about a large fiber deposition area.Contrary, a higher voltage applied to the collector leads to a more intense electric field near tothe collector, i.e., it strengthens the jet movement toward the collector. This suppresses thewhipping movement [7].

5.7. HumidityElectro-spun fiber morphology is influenced by ambient humidity, i.e., interaction between jetsolution and moisture. Humidity can affect fiber diameter by means of modifying the solventevaporation rate. The average diameter of nanofibers decreases with humidity increase. Atrelatively high humidity environments, beaded fibers start to form. Conversely, at relativelylower humidity level, the evaporation rate of the solvent could be high due to a difference inpressure between vapor and ambient air inside the electrospinning chamber. As a result, fibershappen to solidify thereafter producing coarse fibers as compared to fibers generated in relativelyhigher humidity levels [75].

Humidity also affects the surface structure in such a way that, in fast evaporationcircumstances caused by low humidity, porous fibers start to form. This could be explainedby the fact that evaporation takes heat energy out of the jet surface, resulting in a reduction onsurface temperature to a level that can initiate tiny ice formation on the filament surface. Thesetiny ices are preserved until the deposited fibers on the surface of collector exchanges heat withthe ambient air and reaches the ambient condition [76].

5.8. TemperatureTemperature is considered to be in a close relationship with polymer properties affectingcrystallinity and molecular chain orientations [7]. According to Yang et al., the surfacetension and the viscosity of the electrospinning solution decreases with increasing the ambienttemperature. However, an extra increase in temperature causes rapid evaporation of the solvent,which can disrupt the electrospinning process. Therefore, a balanced temperature point shallbe found in order to gain the most desired fiber quality [75,77].

6. Common Polymers in ElectrospinningPolymers used in electrospinning can be natural, synthetic, or copolymer depending on the needsof the manufacturers as well as the availability of the materials. Examples of natural polymersinclude collagen, chitosan, and fibrinogen [8]. Natural polymers have some advantages oversynthetic polymers due to their immunogenicity and biocompatibility. Natural polymers such ascollagen and gelatin can provide solutions used in the electrospinning process. In circumstanceswhere synthetic fibers are readily available, natural fibers may not suffice. Some of thesynthetic polymers include polyvinyl alcohol (PVA), Polyvinylpyrrolidone (PVP), polylactide(PLA), polyglycolide (PGA), poly-D-lactide (PLDA), and polylactide-co-glycolide (PLGA) [57].Combining either natural or synthetic fibers or combining several synthetic fibers can producecopolymers [59].

The aim is to develop polymers that can withstand various limitations, including heat anddegradation. Copolymers are usually developed to overcome the limitations of a given naturalor synthetic polymer [60]. For example, adding poly (glycolide) can minimize the stiffness orrigidity of poly (ethylene-co-vinyl alcohol) (PEVA) [61]. Most copolymers provide a variety offeatures needed by manufacturers to develop suitable nanofibers.

7. Applications of Electrospinning and its Role in Energy Storage SystemsElectro-spun fiber size may exist within the nanoscale while the fibers may have the nanoscalesurface texture that results in various modes of interaction in comparison with the macroscalematerials. The ultra-fine fibers arising from electrospinning have two significant properties,such as a large surface to volume ratio and a structure that is relatively free from defects ata molecular level. A high surface to volume ratio makes electro-spun materials suitable forundertaking activities that require a higher physical contact, including providing a site for achemical reaction as well as the filtration of small-sized physical materials. Also, minimal defectsat a molecular level allow electro-spun fibers to attain maximum strength, thus can attain highmechanical performance for composite materials [78].

There are various applications of electro-spun fiber can act as filters; for instance, theLycopodium moss spores have a diameter of 60 micrometers, thus can only be captured byan electro-spun polyvinyl alcohol fiber. Nanofibers webs may act as an efficient filtering mediumas the nanofibers have small London-Van Der Waals forces that are crucial for adhesion amongthe fiber that captures materials. Nanofibers in textile manufacturing provide an opportunityof developing seamless non-woven garments that may have a variety of functions includingenvironmental, flame, and chemical protection. Electrospinning can combine various coatingsand fibers in order to develop three-dimensional shapes, for instance, clothing that consists ofdifferent layers of polymers [79].

Medical application of nanofibers involves tissue engineering where electro-spun scaffold maybe penetrated with cells that treat or replace biological targets [80]. Also, wound dressing usingnanofibers have an excellent capability to separate the wound from microbial infections [81].Electrospinning is essential in the development of medical textile materials or diverse fibroustreatment delivery system comprising of transdermal patches and implants. Electrospinningcan ensure the establishment of a continuous manufacturing system within the pharmaceuticalindustry. Synthesized liquids can be turned into a tablet using electrospinning [82].

Electrospinning is a feasible process that can be suitable for the manufacture of elongatedcomposite materials within a stipulated timeframe. The process has the potential to producefibers in sufficient quantities within a reasonable period. Research has illustrated thatelectrospinning is the most cost-effective way to manufacture the various medical fibers such asmedical implants, scaffolds, wound dress for artificial human tissues. Scaffolds function similarlylike an extracellular matrix found in natural tissues [83]. Biodegradable fibers are used as anextracellular matrix and may be coated with collages in order to promote cell attachment. The

last application of electro-spun fibers is that they act as catalysts, where they act as a surfacewhere enzymes are immobilized. The enzyme can be vital in breaking down toxic chemicalsfrom the environment [84]. The role of electrospinning is to produce fibers used in energyconversion and storage. The electrospinning process produces fibers with diameters rangingfrom nanometers to micrometers [85]. The fibers provide sufficient storage space and also playsa critical role in converting the stored energy into electrical currents. A general spinning setupconsists of a high voltage power supply, a grounded collector, and a syringe with a metallicneedle [86].

In most cases, the supply of the voltage is either a melt or a solution. As the heatingcontinues, a pendant droplet forms beneath the syringe. The pendant droplet is subjected toelectrostatic repulsion to turn it into a cone-shaped material known as Taylor cone. As theelectrostatic propulsion continues, the conical droplet discharges polymer solution at the tip ofthe needle [87]. The interaction between the electric field and surface tension eventually forcesthe solvent to evaporate, leaving behind a long, thin filament which solidifies into a uniformfiber. Electrospinning began in the 1930s, and experts have used it in the development of fibersused in storing and conversion of energy inside the lithium-ion batteries [88]. Many adjustmentshave been made to the process to make it less energy consuming and more productive.

Some of the advantages of electrospinning include flexible temperatures, short productioncycle, and little pressure. Besides, nanofibers synthesized from the hydrothermal method have alower aspect ratio, which is critical in the transfer of energy [89]. In other words, fibers made fromelectrospinning are likely to provide more efficient energy transfer compared to other methodssuch as electro-spun NWs [90]. However, electrospinning also has various limitations, includingdifficulties in preparing inorganic nanofibers and limited quantity or variety of polymers used inthe process. Limited variety of polymers restricts manufacturers to the use of available materialswhich may not reach the desired energy capacities [91]. Besides, the performance of nanofibersmade from the inorganic materials is likely to decline after calcination. Manufacturers arealso silent about the aging process which renders many batteries inefficient. The aging processdrains the energy capacity of various cells and reduces the performance of lithium-ion batteries.According to Zhang, Tan, Kong, Xiao, and Fu (2015), much research is ongoing to determinethe cause of aging and develop appropriate measures [92].

8. ConclusionThe ultra-fine fibers arising from electrospinning have two significant properties, such as a largesurface to volume ratio and a structure that is relatively free from defects at a molecular level. Ahigh surface to volume ratio makes electro-spun materials suitable for undertaking activities thatrequire a higher physical contact, including providing a site for a chemical reaction as well asthe filtration of small-sized physical materials. Also, minimal defects at a molecular level allowelectro-spun fibers to attain maximum strength, thus can attain high mechanical performancefor composite materials.

The electrospinning process produces fibers with diameters ranging from nanometers tomicrometers [85]. The fibers provide sufficient storage space and also plays a critical role inconverting the stored energy into electrical currents. Some of the advantages of electrospinninginclude flexible temperatures, short production cycle, and little pressure. Besides, nanofiberssynthesized from the hydrothermal method have a lower aspect ratio, which is critical in thetransfer of energy [89]. In other words, fibers made from electrospinning are likely to providemore efficient energy transfer compared to other methods such as electro-spun NWs [90].However, electrospinning also has various limitations, including difficulties in preparing inorganicnanofibers and limited quantity or variety of polymers used in the process. Limited variety ofpolymers restricts manufacturers to the use of available materials which may not reach thedesired energy capacities [91]. Besides, the performance of nanofibers made from the inorganic

materials is likely to decline after calcination. Manufacturers are also silent about the agingprocess which renders many batteries inefficient. The aging process drains the energy capacityof various cells and reduces the performance of lithium-ion batteries.

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